In a manufacturing process for semiconductor devices, a reduced-projection exposure apparatus referred to as a stepper or a scanner exposes and transfers an original image pattern (referred to as a mask or a reticle. The mask and reticle are hereinafter collectively referred to as a mask) in which a circuit pattern is formed, onto a wafer. For LSIs that involve high manufacturing costs, improving the yield in the manufacturing process is essential. A defect in a mask pattern is a major factor that reduces the yield of semiconductor devices. Thus, when a mask is manufactured, an inspection process for detecting possible defects is important.
In the inspection process, not only are possible defects detected but also the line width (CD: critical dimension) of a pattern in a mask surface is measured so as to allow mapping of the distribution of amounts of misalignment (critical dimension errors ΔCD) between the measured line width and the line width in a design pattern. The resultant ΔCD map is fed back to the mask manufacturing process and utilized to review process conditions.
An inspection apparatus in which light having exited a light source is split into two optical paths including an optical path that illuminates a mask as an inspection target with transmitted light and an optical path that illuminates the mask with reflected light is disclosed. The inspection apparatus performs inspection by acquiring an optical image of a pattern on the mask using a sensor on which light transmitted through the mask is incident and a sensor on which light reflected by the mask is incident. In the inspection apparatus, a polarization beam splitter is arranged on the optical path through which the light transmitted through the mask is incident on the sensor. The polarization beam splitter is coupled to the optical path that illuminates the mask with the reflected light. The light reflected by the polarization beam splitter illuminates the mask and is reflected by the mask. The light is then transmitted through the polarization beam splitter and enters the sensor.
In general, the mask includes a glass substrate such as a quartz substrate and a pattern formed of a light shielding film such as a chromium film provided on a surface of the glass substrate. The glass substrate exhibits birefringence, and the directions of the birefringence vary with position due to distortion of the interior of the substrate. Thus, disadvantageously, the polarization state of the light changes at the time of transmission through the mask, and the amount of light transmitted through the polarization beam splitter decreases. Specifically, upon being transmitted through the mask, circularly polarized light turns into elliptically polarized light due to the difference in the directions of the birefringence in the mask surface. The light transmitted through the mask is originally changed, by a quarter-wavelength plate, from the circularly polarized light to linearly polarized light having only a P polarization component with respect to the polarization beam splitter. The linearly polarized light is then transmitted through the polarization beam splitter. However, when the elliptically polarized light is incident on the quarter-wavelength plate, the light entering the polarization beam splitter is not linearly polarized, or linearly polarized light having an S polarization component with respect to the polarization beam splitter enters the polarization beam splitter. As a result, the amount of light transmitted through the polarization beam splitter decreases, varying the measured value of the pattern line width in the mask surface.
In connection with this problem, a part of the quartz substrate in which no pattern is formed is, before defect inspection, illuminated with light so as to allow light transmitted through the substrate to enter the sensor, allowing brightness distribution data to be generated. The brightness distribution data is then used to correct a brightness value in the actual inspection. However, acquiring brightness distribution data before the inspection needs much effort and time. No description of generation of a ΔCD map is disclosed.